This invention pertains to the art of packaging materials, with particular reference to packaging films. More specifically, it relates to synthetic polymer packaging materials made of polyvinyl alcohol used to package materials which are sensitive to oxygen and water vapor.
Currently available synthetic polymers which are used for packaging food stuffs, medicines and related substances, have a substantial rate of oxygen permeability and water vapor transmission so that the foods and medicines packaged in these materials suffer substantial degradation, thereby losing their taste, potency or customer appeal.
Of the various polymer systems which have been considered as barrier packaging materials, polyvinyl alcohol ranks among the best. Polyvinyl alcohol forms tough clear films which are known for their abrasion resistance and high tensile strength. The oxygen barrier quality of polyvinyl alcohol is superior to all other polymers; however, this quality has been subject to severe degradation at high humidities. Polyvinyl alcohols presently known show a rapid loss of gas barrier performance above 50% relative humidity. In addition, these polyvinyl alcohols demonstrate significantly poorer barrier properties below the 95% hydrolysis level.
In general, the use of one polymer in an extruded film does not provide sufficiently long package life to materials susceptible to oxygen deterioration, and multiple-ply films have been developed for these uses. These multiple-ply products are made by post-lamination or co-extrusion near melt temperatures of a number of different types of polymeric materials to provide optimum life to pre-packaged goods sensitive to oxygen deterioration. It has been conventional to laminate a polyvinyl alcohol core between two outer layers of other polymers which provide a structural support for the polyvinyl alcohol core which provides the gas barrier properties.
The literature indicates that films of polyvinyl alcohol (PVOH) have usually been produced from aqueous solutions or in the presence of some water in conjunction with plasticizers. Conventional technology recognized high-boiling, water-soluble organic compounds containing hydroxyl groups as the most effective plasticizers for polyvinyl alcohol (see, e.g. Kirk-Othmer, Encycl. of Chemical Technology, 3d ed., 1983, vol. 23, p. 854). Glycerine and various glycols are the most widely used plasticizers. Glycerine has frequently been used with water, and water/plasticizer mixes were the most common methods for preparing films and formed objects even after the work of Takigawa which resulted in U.S. Pat. No. 3,607,812 in 1971, when he used glycerine in the absence of water. Even in texts published at a later date, practical methods of fabrication involving PVOH invariably use water as a component in addition to the plasticizer in forming extruded products.
Melt processing additives have been used with PVOH to facilitate extrusion because extrusion of PVOH into a usable form is impossible as a dried powder. This inability to extrude arises from the melt temperature of PVOH being very close to its decomposition temperature, with the rate of decomposition being time and temperature dependent. It will decompose at the melting temperature of about 232.degree. C., if exposed to this temperature for a short period of time. PVOH will also decompose at lower temperatures when left for significantly longer times, even at temperatures as low as 180.degree. C.
It is common in the art to provide additives to decrease the melt temperature of the PVOH system thereby avoiding or minimizing rapid decomposition. It is also known that a PVOH which contains over 5% unhydrolyzed polyvinyl acetate, or which contains a hydroxyl containing plasticizer, will melt at a lower temperature. Thus, the melting temperature of PVOH homopolymer is 232.degree.-235.degree. C. while the melting temperature of a PVOH copolymer containing 12% unhydrolyzed polyvinyl acetate is 195.degree. C. The addition of about 10% glycerine, a hydroxyl containing plasticizer, to either the homopolymer or the copolymer lowers the melting temperature 7.degree. to 10.degree. C. This is one reason why glycerine and various glycols are the most widely used plasticizers.
Formed products are presently fabricated using water or a water/plasticizer combination so as to decrease the melt temperature of the PVOH and thereby avoid or minimize rapid decomposition. Homopolymers of PVOH, copolymers with methyl methacrylate (up to 6%), and copolymers with vinyl acetate (up to 15%) have been extruded with water/plasticizer mixtures by removing the water prior to the polymer leaving the die as a melt or by using a special extrusion technique.
PVOH is prepared by the hydrolysis of polyvinyl acetate (PVAc). Until the early 1970's, the term "polyvinyl alcohol" was used in the literature to describe both partially hydrolyzed PVAc and PVAc which has been substantially fully hydrolyzed to PVOH. It is now conventional to describe materials which are at least 95% hydrolyzed as PVOH homopolymer and those which are less than 95% hydrolyzed as PVOH/PVAc copolymer.
Since the homopolymer and the copolymer are not equivalent in all their properties, and in particular, demonstrate different water solubilities at ambient temperatures and different permeabilities to gases such as oxygen and carbon dioxide, early references to PVOH may be misleading. It is important to identify the degree of hydrolysis of the material. Homopolymer and copolymer cannot be treated as being equivalent.
Exemplary of the many water containing polyvinyl alcohol compositions is Pockel, U.S. Pat. No. 2,963,461, (Dec. 6, 1960) who discloses a novel hydroxyl containing plasticizer for polyvinyl alcohol. Primarily interested in cast polyvinyl alcohol in which water is added to the polyvinyl alcohol system, Pockel also discloses extrusion of polyvinyl alcohol in systems where no water is added to the polyvinyl alcohol/hydroxyl-containing plasticizer system. Depending on the relative humidity of the environment, the highly hygroscopic polyvinyl alcohol used would have contained from about 5% to about 15% water.
The following references exemplify the art relating to the extrusion of PVOH in the absence of water.
The 1965 work, Physics of Plastics, edited by P. D. Ritchie, discloses, at page 350, that polyvinyl alcohol is normally plasticized when used to produce sheet and rod. It further discloses that glycerol (hydroxyl group containing) and p-toluene sulphonamide (non-hydroxyl containing) are suitable plasticizers. No mention is made of drying the composition before extrusion.
The first clear description of a substantially anhydrous (less than 2% water) system does not appear until 1971, in Takigawa et al, U.S. Pat. No. 3,607,812, (Sept. 21, 1971). A conventional polyvinyl alcohol is utilized with a hydrolysis degree of at least 97% and a polymerization degree of 700-1500. The plasticizer is a polyhydric alcohol and the extrusion is done at moisture contents of less than 2%.
Coker, U.S. Pat. No. 3,977,489, (Dec. 14, 1976) discloses combinations of waxes and fatty acid derivatives to aid in the melt flow of polymers such as those disclosed by Takigawa. Coker's melt extrudable PVOH contains:
(1) a hydrocarbon oil or wax, (2) a polyethylene wax or ethylene polymer, and (3) optionally, a plasticizer. Hydroxyl and non-hydroxyl containing plasticizers are disclosed and no reference is made to the presence of water.
Yamata, Japanese Pat. No. 52-65548, (May 31, 1977) discloses a polyvinyl alcohol composition prepared from at least 70% hydrolyzed polyvinyl alcohol having a polymerization degree of between 500 and 3000 and a specified plasticizer, both of which may have been dehydrated before mixing.
Yamata's plasticizer may be non-hydroxyl containing (aromatic sulfonamides) or hydroxyl containing (p-hydroxybenzoic acid esters) and is present in amounts ranging from about 5% to about 67% by weight of the composition. Yamata's plasticizer is claimed to provide a PVOH material which has lessened solubility in water or heated water, lessened volatility during melt molding and lessened sweating (the tendency of the plasticizer to migrate to the surface which then becomes sticky at high temperatures and humidities).
Three of the references discussed above disclose non-hydroxyl containing plasticizers: Physics of Plastics in 1965, Coker in 1976 and Yamata in 1977. In each case, the reference also discloses hydroxyl containing plasticizers and does not distinguish between the two types of plasticizers. These references teach that there is an equivalence between hydroxyl containing and nonhydroxyl containing plasticizers for polyvinyl alcohol and that the two types of plasticizers are interchangeable.
Polyvinyl alcohol has been blended with copolymers to provide properties enhanced for special applications or to solve certain problems. In all cases, these references use conventional polyvinyl alcohols and do not disclose the use of nonhydroxyl containing plasticizers in conjunction with an anhydrous environment to provide enhanced gas barrier properties.
Thus, Schroeder, U.S. Pat. No. 4,254,169, (Mar. 3, 1981) discloses multi-layer films with a core layer of polyvinyl alcohol or ethylene-vinyl alcohol copolymer prepared by conventional means.
European patent publication No. 0,063,006 (Oct. 20, 1983) discloses ethylene-vinyl alcohol copolymer blends and a process for manufacturing films from such copolymers.
It is apparent from the above review of the background of the invention and the state of the art, that completely hydrolyzed polyvinyl alcohols and copolymers of polyvinyl alcohol with polyvinyl acetate and other polymerizable monomers and in particular olefin modified polyvinyl alcohols have outstanding usefulness because of their very low gas transmission rates, at low relative humidities, particularly for gaseous oxygen. It is well known in the packaging art that the principal cause of packaged goods' deterioration with storage age is due to oxidative deterioration. Thus, the art is replete with attempts to produce films and molded articles utilizing polyvinyl alcohol as at least one layer of a packaging material to inhibit oxygen transmission through the packaging material at both low and high relative humidity levels.
These attempts have not been successful. There is today no satisfactory polyvinyl alcohol homopolymer composition which can be formed by conventional techniques and which exhibits a desirable level of impermeability to gases at high relative humidities.